The present invention generally deals with satellite communications. A user terminal communicates with other terminals or with a gateway that gives the user terminal access to internet, by way of a satellite. A user terminal may stay in one position or may be mobile e.g., satellite phones or computers capable of communicating with a satellite.
In order to effectively communicate with a satellite in some systems, a user terminal must know where to transmit its signal. In other words, a user terminal needs to know both its position and the position of the satellite to effectively communicate with the satellite.
In situations where the user, terminal does not move, the user terminal is generally provided with its position information. In some situations, satellite position information may be provided to the user terminal by the ephemeris data, which provides orbital and time information of communication satellites that are orbiting around the world. Based on this information, the user terminal will know where and when a satellite is going to be in a position to communicate. The geographic location information of the stationary user terminal is known and does not generally need to be updated. So, communication between the user terminal and the satellite communication system can be easily established based on the known position of user terminal and the known position of the communication satellite. But this becomes complicated when the user terminal is moving.
In the situation where the user terminal is moving, the user terminal has to constantly update its position information. Conventionally this is done by receiving position information from GPS satellites. A GPS satellite continuously transmits signals that provide time information and the satellite's orbital information, called ephemeris data. A GPS receiver within the user terminal may calculate position information based on the transmit time of the received signal. It uses this information to compute its distance to a plurality of satellites within a GPS constellation. These distances along with the satellite's position are used to compute the position of the receiver.
Typically, in conventional satellite communication systems, a user terminal that communicates with the communication satellite needs to know its own position. Generally, this position information is provided to the user terminal by the GPS satellite system. There are times though that a user terminal may lose communication with the GPS satellite system and therefore be unable to determine its position. As such, because the user terminal cannot determine its position at some points, the user terminal loses communication with the communication satellite. This will be further described with reference to FIGS. 1A-1C.
FIG. 1A shows a wireless communication system at a time T0.
As shown in the figure, the system includes a communication satellite 100, a GPS satellite 102, and a car 114 housing a user terminal 116 (not shown) at position 104.
Communication satellite 100 is operable to communicate bi-directionally with user terminal 116 within car 114 by way of communication channel 106. GPS satellite 102 is operable to send GPS signals to user terminal 116 within car 114 by way of communication channel 108. For purposes of discussion, only one GPS satellite is shown. However, one of ordinary skill would recognize that the user terminal is going to be able to communicate with a constellation of GPS satellites, although only one is represented here.
In a typical situation, user terminal 116 in car 114 communicates with communication satellite 100. In order to do this, user terminal 116 needs to know its GPS position. User terminal 116 may obtain its GPS position from GPS satellite 102 via communication channel 108.
There may be situations where user terminal 116 is unable to receive signals from GPS, satellite 102 because, for example, the car goes into a tunnel, trees or clouds block the signal. In other situations, the user terminal may be carried on a person, where the person might go inside a building. The state of the wireless communication system when the user terminal is unable to receive signal from GPS satellite 102 will be described with reference to FIG. 1B.
FIG. 1B shows the state of the wireless communication system of FIG. 1A at time T1, where car 114 is at position 110.
As the car moves from position 104 to position 110, the car is still able to communicate with communication satellite 100. But, as discussed above, the car has lost the signal from GPS satellite 102. In this example, user terminal 116 is in car 114.
When user terminal 116 is unable to determine its own position, for example with a loss of GPS information as a result of a loss of communication with GPS satellite 102, user terminal 116 will be unable to communicate with communication satellite 100. This will be described with reference to FIG. 1C.
FIG. 1C shows the state of the wireless communication system of FIG. 1B at a later time, time T2, where car 114 is at position 112.
As the car continues to move from position 110 to position 112, car loses communication with both, communication satellite 100 and GPS satellite 102. User terminal 116 loses communication with GPS satellite 102 as discussed above with reference to FIG. 1B. As a result user terminal 116 is unable to determine its own position. Without position information provided by the GPS satellite 102 user terminal 116 is unable to transmit to by satellite 100.
What is needed is a system and method that enables a mobile user terminal to communicate with a satellite without relying on GPS.